1. Field of the Invention
This invention relates to the fields of optical and electromagnetic communications and cryptography.
2. Description of the Background
Sender-receiver units for communication decode information at the sender end based upon expected encoding format at the receiver end. Methods for encoding information use the polarization of an optical signal and the phase between time-separated components of an optical signal, respectively. Quantum cryptography is disclosed in C. H. Bennett and G. Brassard, “Quantum Cryptography: Public Key distribution and coin tossing,” in Proceedings of IEEE International Conference on Computers, Systems, and Signal Processing, IEEE, pp.175-9 (1984), and referred to herein as “BB84.” BB84 premises a protocol named quantum key distribution, which employs two sets of quantum states with two members in each set. The two members of each set are orthogonal to one another, but the members of one set are not orthogonal to the members of the other set.
The methods of encoding information mentioned above translate into two basic categories of effective orthogonal states, polarization and phase, that have been developed for establishing a quantum key by transmitting optical qubits through free space, optical fibers, or other media. Qubit is the name for the encoded quantum degree of freedom, here polarization or phase. In the polarization category, weak optical signals, ideally single photon states, are sent from the sender Alice to the receiver Bob [C. H. Bennett and G. Brassard, IBM Technical Disclosure Bulletin 28, 3153 (1985)]. A sufficient choice for the four polarization directions might be the set of right and left circularly polarized light fields plus the set of linearly polarized light fields making angles ±45′ with respect to the horizontal and vertical. (Note that optical polarizations in uniform media are normally defined with respect to the propagation direction of the light and the plane perpendicular to that direction. The horizontal and vertical directions lie in that perpendicular plane.) These four polarization vectors can be written in a compact form in terms of a phase difference between horizontal and vertical polarization directions,
  h  ⊥and
      𝓋    ⊥    ,as
                    ɛ        r            ⁡              (        θ        )              =                  h        ⊥            +                        ⅇ                      i            ⁢                                                  ⁢            θ                          ⁢                  𝓋          r                      ,where θ=−π/2, π/2, 0, and π for right circular, left circular, 45° linear, and −45° linear polarizations, respectively. Thus, the information in a polarization-based signal is uniquely given by this phase θ. In quantum cryptography, Alice makes a random choice of the polarization of her signal, while Bob selects a polarization basis at random for detection. The process allows for the distillation of a secure quantum key. Crucially, Alice and Bob have devices designed to send and process only the quantum information contained in the polarization of the light signals.
In the phase category of quantum key distribution, Alice encodes the quantum information in the random phase of a signal she sends to Bob [C. Bennett, PTO/US5307410; P. D. Townsend, J. G. Rarity, and P. R. Tapster, Elect Lett 29, 1291 (1993); A. Ekert et al., “Quantum Cryptography”, in D. Bouwmeester, A. Ekert, A. Zeilinger, The Physics of Quantum Information, Springer-Verlag, Berlin, p. 32 (2000)]. Using an interferometer with unequal path lengths and an input field pulse envelope E˜f (t) in time, Alice creates an outgoing electromagnetic field signal of the form E(t)˜(f(t−r)+etθf(t))/√{square root over (2)}, where θ is the relative phase difference between outgoing field pulses that is imposed by an (active) phase modulator in one of the interferometer paths, and τ is the time delay of one of the interferometer paths relative to the other. The input field polarization here is not specified since it contains no information. Here, all of the information is contained in the imposed phase θ. To implement BB84, the active but random choice among the phases 0, π, and ±π/2 by Alice here is analogous to the choice of four polarization directions in the polarization-based scheme above. In the phase-encoding case, Bob's measurement consists of imposing a phase 0 or π/2 randomly on the signal at his receiver end in his own interferometer. Constructive and destructive interference of the combined phases imposed by Alice and Bob allow a secret quantum bit to be established. Again, Alice and Bob have devices designed to send and process the quantum information only as phase-encoded signals.
The quantum cryptography work of Chiangga and coworkers used 853 nm optical pulses of approximately 10 ns duration at a 2 MHz repetition rate to encode, send, and receive polarization qubits. See S. Chiangga et al., Appl Phys B 69 (1999).
K. J. Blow, R. Loudon, and S. J. Phoenix, Phys Rev A 42, 4102 (1990) describes propagation of a single photon, that is an eigenstate of the number operator with eigenvalue N=1 for the longitudinal modes in quantum optics.
W. Tittel, G. Ribordy, and N. Gisin, “Quantum Cryptography”, Physics World 3, 41 (1998) describes a phase-encoded quantum cryptography scheme.
P. Townsend, PTO/US5953421, discloses two schematics. One assumes a polarization-based sender and receiver. The other schematic assumes a phase-based sender and receiver.
S. Chiannga et al., Appl Phys B 69 (1999) discloses use of expensive devices for phase modulators or active phase manipulation in order to generate polarization qubits.
C. Santori et al., Phys Rev Lett 86, 1502 (2001) discloses that triggered true single photon sources may emit photons along only a particular polarization direction.
PCT publication WO 9744936, naming Townsend as inventor, discloses a phase-based quantum cryptography scheme.
W. Tittel, G. Ribordy, and N. Gisin, Quantum Cryptography, Physics World 3, 41 (1998) discloses how to avoid instability of separated interferometers.
U.S. Pat. No. 5,768,378 to Townsend discloses employing quantum cryptography in a passive network environment.
U.S. Pat. No. 6,028,935 to Rarity et al. discloses a passive quantum cryptographic detector set-up.
The teachings of N. Gisin et al., http://xxx.lanl.gov/abs/quant-ph/0101098, are hereby incorporated by reference.
The present inventors realized that polarization signals coupled to a transmission channel may require active stabilization control owing to polarization rotations through optical elements, birefringence, polarization mode dispersion, and thermal and mechanical fluctuations.
The present inventors realized that a sender may want to create deterministic or random polarization-based signals, while the receiver may want to receive a phase-encoded signal. The present invention provides this capability by allowing polarization-based signals to be converted into phase-encoded signals for transmission and/or reception. By assuming a polarization-based signal is what is input into a system of the present invention, the system also eliminates the active phase modulation required for phase-based senders.